Officially known as an anal stretching ring. A device used for that purpose, to stretch - and keep it loose enough - one's anus in preparation for anal sex. Serves the same purpose as a anal/butt plug. Comes in various sizes and can be made from different materials.

The doctor used an anal ring to prep me for my enema. (read this in a book)

When your new fiancee lets you give her anal after proposing for marriage.

Dad: Hey son. She is going to give you the anal ring for proposing! Your mom gave me one!
Son: DAD! Come on!

Woman 1: I think I need to start stretching my anus. I think Tom is going to ask me to marry him this weekend.
Woman 2: Then you better get to stretching. An anal ring is precious but can be painful if you don't plan right.

The definition of Anal Ring, closely resembles the actual words which is describing. The Anal Ring is the skin that is between the outer side and the inner side of the anus. It is just the circular gateway of your buttock.

The Anal Ring can be open and closed, by means of contraction and relaxation of the sphincter. A rounded-shaped muscle which encircles the Anal Ring.

To a girl, exploring her Anal Ring can be close to as orgasmic as finding her G-Spot, also known as "Grafenberg spot"